The invention relates to a transmitter for transmitting frequency-modulated signals, comprising a pre-modulation filler for generating from an incoming data signal a data signal which at instants t=nT has at least five signal levels, and a frequency modulator connected to the premodulation filter.
Such a transmitter is described in the article "Tamed Frequency Modulation, A Novel Method to Achieve Spectrum Economy in Digital Transmission" by F. de Jager and C. B. Dekker and published in "IEEE Transactions on Communications" Vol. Com. 26 no. 5, May 1978. The modulation described in this article relates to what is commonly referred to as tamed frequency modulation.
Frequency-modulation is preferably employed in radio communication systems because of the constant amplitude of a frequency-modulated carrier, which enables high-efficiency amplification by means of non-linear amplification. A disadvantage of frequency modulation is however that the spectrum is rather wide. In order to reduce this spectrum in digital signal transmission, the data to be transmitted is preprocessed by employing a certain intersymbol interference, whereby signals with pseudo multi-level signals are obtained, before they are applied to the frequency modulator.
So, in accordance with the above-mentioned article for the generation of a TFM signal a pseudo fivelevel signal is first assembled from an incoming data signal in accordance with the code rule: .pi./2(a.sub.n-1 /4+a.sub.n /2+a.sub.n+1 /4), wherein a.sub.n represents the n.sup.th bit of the incoming data signal with n=1, 2, . . . . It is filtered by means of a Nyquist III filter, realized by means of a Nyquist I raised cosine filter, and thereafter modulated on a carrier signal in a frequency modulator.
By means of such a preprocessing operation, or a similar one, the required bandwidth is indeed significantly reduced, but a pseudo five-level signal requires a comparatively complex detection circuit at the received side and normally requires coherent detection.